Telephone repeater circuit



K. s. JoHNsoN 2,408,072

TELEPHONE REPEATER CIRCUIT Filed Dec. 31; 1943 -3 Sheets-Sheet 1 LMI R AQN R 58.

' NLT /NvENroR A KS. JOHNSON vBi B. C. @UVA AT TORNEI Sept. 24, 1946.

Sept. 24,1946V Kr s. JHNsoN 'A 2,403,072-

ITEpEPHoNE REFEATE'R CIRCUIT Filed Dec. s1. 194s s 'sheets-sheet 2 INVEN TOR ATTORNEY Y' Patented Sept. 24, 1946 TELEPHONE REPEATER CIRCUITKenneth S. Johnson, South Grange, N. J., assignor to-Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication December 31, 1943, Serial No. 516,464

16 Claims. l

e This application is in part a continuation .of my ccpendingapplication Serial N o. 509,056, led November 5, 1943, for Telephonerepeater circuits.

This invention relates to systems, as for example, telephone repeatersystems, involving networks that include conjugate circuits and have abranch subject to impedance variation that disturbs the conjugacy.

An object of the invention is to maintain conjlugacy in such networks. f

In one specic aspect the invention is a system comprising a cord circuitrepeater for connection.

to any chosen one of a groupof subscriber loops of various impedances,and having direct current supplied from the cord circuit over theconnected loop for talking or supervisory purposes, the cord circuitrepeater including therrnistor means responsive to the direct current toso adjust the impedance of a balancing circuit for the loop as toestablish impedance balance between the loop and the balancing circuit.

Other objects, aspects and features of the invention will be apparentfrom the following description and claims.

Fig. 1 shows a system in which the invention is applied to a cordcircuit repeater of the 21-type;

Figs. 2 to 6 show four-terminal networks useful in systems such, forexample, as those of Figs'. 1 and '7; and

Fig. 7 shows a system in which the invention is applied to a cordcircuit repeater of the 22-type.

In Fig. 1 is shown a cord circuit or connecting circuit, comprising a.2l-type repeater including amplifier I and bridge transformer (hybridcoil) 2, for connecting any chosen one of a group 3 of a considerablenumber of subscribers loops such as the two indicated at il and 5 withanychosen one of a group 6 of a considerable vnumber of subscribersloops such as the two indicated at 'l and :8. The loops. are shownterminated in substation sets 9, I0, I I and I2, which may each have animpedance of 60 ohms, for example.

Switching means of any suitable type, as for example plug I3 forcooperation with jacks of loops 3 and plug I8 for cooperation with jacksof 'lines 6, may serve to connect the cord circuit with loops 3 and 6. Y

The loops 3 have their impedances facing the cord circuit approximatelyequal. For example,

they may lbe private branch exchange loops of negligible length, or maybe loops in a congested business area, all of substantially the samelength.

The loops 6 may have various lengths, or substantially differentimpedances facing the cord circuit; and, indeed, any one may beconsidered to include one or more interoffice trunks of considerablelengths (switched in attimes in extending the talking connection fromthe cord circuit to a desired subscriber station) so that its impedancefacing the cord circuit suifers substantial variation.l

The cord circuit includes a link 2| connecting hybrid coil 2 with plugI3, and a link 22 connecting the hybrid coil with plug I8. The link 22includes supervisory relay 23 and central oice wiring represented by itsequivalent resistance 2li. Two thermistcrs 25 and 2S comprisetemperature-dependent resistances T11 and T (which may have negativetemperatures coeii'lcients of resistance) and heater elements I-I1 andH2 electrically insulated from VT11 and T. (Thermistors are described inan article by G. L. Pearson at page 106 of the Bell Laboratories Recordfor December 1940.) The elements H1 and H2 are in series in the link 22.Resistance T is connected in series in the link 2I through animpedance-modifying network 3I and an impedance transformer 32. The(variable) resistance of element T is designated T and that of T11 isdesignated T11. The resistance presented tc the transformer 32 by thenetwork 3I terminated rby T is designated R. The resistance inserted inthe link 2| by the transformer 32 is designated r. This resistance formsthe series arm of a 1r-network in the link 2|, one ofV the shunt armsincluding a group of condensers 33' adapted to be connected in parallelby a sliding switch 34, and the other shunt arm including a group ofcondensers 35 adapted to be connected in parallel by the switch 34.

The switch 34 is shown drawn to its extreme right-hand position againstthe tension of the spring 36, by action, on its soft iron plunger 3l, offlux due to current in its operating coil 38. This crurrent maybe spacecurrent of a tube 4I, controlled by temperature-responsive resistanceT11. As described hereinafter, this current can -be reduced by heatingT11. Then spring 3S can draw switch 3'11 to the left. The switch in thismovement to the left, can remove from circuit the condensers 33successively, and simultaneously remove from circuit the condensers 35successively. In the extreme left-hand position of the switch, all ofthe condensers 33 and 35 will be out of circuit. Y

Direct current is 'suppliedto the substationY telephone sets connectedto the cord circuit from battery 45 (of 24 volts, for example) throughthe connecting loops. This direct current applied over the loops to thesubstations may be either 3 for talking and supervisory purposes, if thesubstation is of the common battery type with its transmitter energizedfrom battery Q5, or for supervisory purposes only, if the substation isof the local battery type with common battery super- Vision. For thesets connected to loops 3, this direct current can be supplied Via link2l and the.

loop 3 to which plug I3 is connected. For the sets connected to loops 5,the direct current can be'sup-` When plug E8 is disconnected from allofthe' loops 5, no direct current flows through heaters Hz and H1. rEhenT, R, and 1' have their maximum values. T11 also has its maximum Value,so the current flowing from battery i9 throughA resistor 5i! has itsminimum Value. Consequently, the` component of negative grid biasthenfurnished for tube lll by the voltage drop across resistor due tothis current in 59,' has its minimum Value; and the space current Voftube #it flowing in coil 33 has its maximum value, so switch 3d is inits extreme right-hand position', as shown.

When the kcord circuit is connected between two loops, itis'desired'that before switch El is closed to complete the repeatercircuit, the link 2l automatically be given'any adjustment re'- quiredfor building out the impedance of the connected lcops Btc make theimpedance e2 presented to terminals 53 of the hybrid 'coil equal theimpedance Z2 presented to terminals 52 of the hybrid coil or balance theimpedance Z2 sufliciently well to prevent the repeater from having anyundue singing tendency.

This automatic adjustment is eiected by response cf thermistor 26 andnetwork' 3l to give r the proper value, and response of thermistor 25,tube il and switch '3d to properly adjust the capacities 33 and 35connected in circuit.

The longer the loop 6 to which plug IB is connected, the greater willvbe its resistance, the less will be the current in'Hz, and the 'greaterwill be T, R and 1; and the shorter the loop, the less will' be itsresistance, the greater will be the current in H2, and the'less will beT, R and r. The responsiveness of T and 3l must, by proper design, bemade such as to always establish and maintain lwithin the limits oftolerance the balance between the resistance component of Zz and that ofe2.

In some systems, as for example, where the variation of length of theloops 6, or the range of impedance variation of the loops 6, does noteX- ceed certain limits, the adjustment of resistance balance may beadequate for preventing singing, without necessity forprovision ofcapacities 33 and 35; and then switches 55 may be opened, or capacities33 and 35 and switch 34 and its control circuits may be omitted.However, their pro- Vision' enables closer `impedance balance between e2and Z2 to be obtained, especially in systems in which the Variations inlength or impedance of loops 6 are great. v

If plug I8 be connected, for example, to a loop 6 that is so long or hassuch high resistance and effective shunt capacity (distributed capacity)that, with switch 34 inits extreme right-hand position, the degree ofbalance between Z2 yand z2 is within the limits of tolerance, then there- 'spense of T11, 4I and 34 should be insufficient to disconnect anyof the capacities 33 or 35; whereas, if plug i8 be connected to a loop 6so short as to require a given reduction of the capacities 33 and 35 incircuit in order to bring the degree of balance between Z2 and e2 withinthe limits of tolerance, then the response of T11, 4I and 3eshould besuch as to eiect such reduction.

Network 3| may be referred to as a shaping network. It controls theshape of the characteristic (not shown) of R versus T, as discussedhereinafter. In the specic form shown in Fig. l, it ccnsists ofresistances A and B; but other forms may be used, as willbe madeapparent hereinafter.

Any suitable number of condensers 33 and S5 in link 2lV may be used, toobtain any desired number of steps in the capacity Variation. Theindividual condensers 33 and 35 may have any suitable Values, either thesame or different, to obtain equal or different steps of any desiredmagnitudes; or any suitable form of condensers may be used as forexample, well-known types of ganged air condensers (not shown) with setsof Xed-and movable plates shaped to continuously vary the capacity asdesired upon relative motion of the plates.

Opening switches 55 and closing switches and 6l (with switches B2closed) replaces vthe circuit of tube 4I by a circuit comprising seriesinductance L and shunt capacity C1, an oscillator or other alternatingcurrent source 53 or frequency f, thermistor SS, network 61, and sourceof electromotive force 69. Thermistor 6% comprises atemperature-dependent resistance Te with temperature coeicient ofresistance of the same sign as that of temperature dependent resistanceT11, and heater H6 electrically insulated from resistance Ts. Heater Heis in series with source 63 and condenser C1, which is shunted acrosselements L and T11 in series. Element Ta terminates network 61. Network61 is shown as a 1r network of resistances A, B and C. It is a shapingnetwork for controlling the shape of the characteristic of Re versus T6,where Re designates the impedance or resistance that coil 3S faces (withswitches 55 open) and Ts designates the impedance or resistance ofthermi'stor element T6.

With L and C1 given such Values that 211e tout) at the oscillatorfrequency j or lZv] becomes proportional to the reciprocal of T11. Thatis, at the frequency of anti-resonance the network consisting orresistance T11 and inductance L inseries, shunted by capacity C1, hasits impedance increasein magnitude as the resistance T11 decreases.Thus, increase of the heating current in H1 -causesdecrease of theresistance T11, with consequent increase of [Zf/I, decrease of heatercurrent in Hs, increase of resistance Ts, increase y of resistance Re,and decrease ci?v the current flowing from'battery 69 through vcoil38,.v

Opening switches 50 and 62 1 and v'closing switches 1U and 12 replacesthe source 6'3 and the networkv L, Ciby a source ,13' with resistance'I5 and a lattice or bridge network 14 comprising resistances A, B, Cand D proportioned so that A/B=C/D. Then, as the heating current in H1increases, the resistance T11 becomessmaller-as compared with A-and thelattice or bridge network 14- becomes more Vnearly balanced, so thecurrent owing from source 13 in heater He decreases and consequently theresistance T6 increases. The source 13 may be either a direct currentsource or an alternating current source.

The shaping networks 3| and 61 are typical of four-terminal networksdesigned so that if terminated in a, variable resistance, for example aVnetwork such for instance as that'of Fig. 3, kand three preassignedvalues, which may be designated R1, R2 and Rs can be obtained with athreeelement ladder network such', for instance, as network 61 of Fig. 1vor the network of Fig. 4.

The design formulae for a network of theconfiguration of network 3| canbe shown to be:

A- 2 i 2 t TVT, "R112 and Y Enna- Raw f (2,

The formulae for a network ofthe form shown in Fig. 2 may be given as:

The formulae for a lattice network of the configuration shown in Fig;` 3are:

v(ERz) would be 342 ohms.

The formulae for a ladder network of the form shown in Fig.`4 (a`1r-network) are:

lll

The 1r-network may be replaced by its equivalent T-network with theconversion relations of Figs. 28A and 28B of Appendix D of my bookTransmission Circuits for Telephonie Communication/ published by the D.Van Nostrand Company, New York.

It may be noted that although Formulae Y1 to l5 were derived on theassumption that all of the circuit elements involved were pure'resistances these formulae are also Valid if all of the circuitelements are pure reactances of the same sign. Consequently, if A, B, C,R. and T in the formulae were all replaced by correspondinginduct'ances, (In, Ln, Lc, LR, and LT) kor by the reciprocal ofcapacitances, (I/CA, l/CB, l/Cc, l/CR, andi/Cr), the formulae wouldstill be valid. For example, Formulae l@ to 15 would serve if, with avariable terminating inductance Lr (instead of a variable resistance T),it were desired to design a network composed of pure inductances LA, LBand Lc such that the inductance at the input terminals, LR, would haveanyv three desired preassigned values.

Specific numerical examples of designs for networks of the ladder andlattice types shown in Figs. 5 and 6 are givenfin the following table:

Fig. Re Rx R2 Rs To Tx Tn Ta A B C All the circuits of the table weredesigned such f that (1) `when 860 ohms resistance (=Ta) was connectedacross the output terminals, the input resistance (E123) would be 642ohms, and (2) when 300 ohms resistance (=T2) was connected across theoutput terminals, the input resistance For the cases v1n which A, B andC are all nite, thev design also assumed T1==43 ohms and R1 waspreassigned to have the Values indicated. For the other cases, R1' wascalculated for T1=43 ohms, and for all cases Ro was calculated when theterminal resistance Tb wasl 0 ohms. l

It may be noted that cases 1) andv (f6-)f are -not allegare,

I'J'hysicalflyv realizable sinceeither B or C f is negative. I A

Also, it is noted that the value of Ro (=102 ohms), in the case of thethree-element physically realizable network, (=Case 4) lies between thevalues of R1 (91 and 115 ohms) given by the two-element networks -Case 2and 5); and likewise the value of R1 =142 ohms), in the Case 4, liesbetween the values of R1 v(133 and 152' ohms) given by the two-elementnetworks (=Case Zand 5) For the circuit of Fig. 2, the table below givesthe calculated values of Rand r-for three values of T produced bythree-values of direct current I that flows in heater H2 for threespecied values of RIA-34. RL designates the variable direct currentresistance of the'loop-.circuit connected to link 22 by plug le;and'iill is the Value, in ohms, assumed for the total direct currentresistance of 4.the elementsi-L 2li, H1v and H2 in series in the link22, as indicated by the resistance values appearing in Fig. l. Thedirect current resistance of the substation sets is assumed to be 60ohms as indicated in Fig.V irand I Yis calculated as 154-i-RL because,as indicated in the Fig. l, the voltage of battery 45 is Yassumed to be24 volts, the direct current resistance of they secondary winding of theamplier output transformer is assumed to be 20 ohms, and that of thehybrid coil windings traverseduby I is assumed to be 40 ohms.

RL R11-F32 I T r 0 32 0. 160 6l 20 100 132 :096 287 96 200 232 069 615205 `300v 332 .A053 1000 333 400 432 044 1270 417 500 532 037 1470 490As indicated by the degree toA which) approaches RL+32 the network 3Lmay well be omitted as unnecessary in some cases.

Y Fig. 7 illustrates application of the invention to a system comprising'a cord circuit repeater of ZZ-type including ampliiiers i and I' andhybrid coils 2 and 2', for connecting any chosen one'of a group 6 ofSubscribers loops or trunk circuits such as the two indicated at 1 and 8with any chosen one of a group 6 of subscribers loop or trunk circuitssuch as the two indicated at 1 and 8'. To make such a connection, a plugI8 at one end of the cord circuit is inserted in the cooperating jack ofthe chosen one of lines 6 and 8 and plug I8 at the other end of the cordcircuit is inserted in the cooperating jack of the chosen one ofulines6. The linesv and also the lines G' gmayljlaveyaripus lengths orsubstantially dierent impedances facing` the cord circuit.

The. cord circuit includes a balancing ne"- work or circuit N attachedto the network terminals 53 of the hybrid coil 2. The network Ncomprises k av linkvcircuit. 8i .terminated in a dummy telephonev set oran ,impedance 88 simulating the impedance of a set such as il or l2. Thelink 8l .as shown-fis Vlike link 2l of Fig. 1, except that fan optionalblocking condenser 85 and a-resistance, ,which are referred tohereinafter, are shown in the link 8|.

The cord'circuit includes alink -82 Vconnecting the-line terminals 520ithe-hybrid coil 2 with the plug I8.- The link 8 2.as-shownis like link22 of Fig@ 1,A except that the supervisory relay 23 andcentralloiiceryvirir-ig resistance 24 are omitted land aheater elementH9 of an additional ther@V mistor 90, similar to the thermistors 25 and26, isshown in the link 82. The-thermistor 80comprisestemperature-dependent resistance T9 heatvat ed vby jI-Ig and--electrically Y insulated from H9. Thev resistance T9 may have anegative temperature coeicient of resistance, Yfor example. It isconnected,` across the input circuitofamplier l, either directly orthrough-ashaping network 81shownjas;oi the; same type as network 61, andcontrolsthe gain of that amplifier as indicated.h`ereinafter. Ifdesired, the network 8l maybe as'haping network of the type of network3|v,ormay be of any of the types shown in Figs. 2 tov 6,'for example.` yy The lowerffhalf of the cord circuit repeater as shown in Fig. 7 isladuplicate of the-upper half. The reference characters designating the`elements in theLfloWer ,halfv-'a-r'those designating thecorresponding-elements of the upper half primed. In Fig. 7 the dottedlines connecting thermistor T11 and coil 3 8 are for indicating that theconnection maybe as in vl g..l. Similarly the connection betweenTn' and38 may include the apparatus shown in Fig. l in the connection betweenT11 and 38. Direct currentfrom battery 45 is supplied over theI loop 8to which plug I8 is connected, to thesubstation of that loop, either fortalking and supervisory purposes if the substation islet the commonbattery type, or for supervisory purposes, only, if the substation is ofthe local battery type withV common battery supervision. Similarly,direct current is supplied over the loop 6 to which plug I8' isconnected, to .the substation of that loop. The blocking condenser is ofnegligible-impedance at voicefrequencies. It prevents direct currentflow from battery 45 to network N. This results in a maximum variationofr the resistance T, T11 and T9 as a function of the loop length.Similarly, condenser 85 prevents Iiow of direct current from battery G5to network N Resistance 86 serves to counterbalance the resistance ofheaters H1, H2 and H9; and similarly resistance 88 counterbalances theresistance of H1', H2 and H9. l

The gain which -it is possible to obtain from a 22-type repeater dependsuponthe closeness of lthe impedance balances between (a) the line in onedirection and its balancing network, and (b) the line in the otherdirection and its balancing network. In the case of each of the twobalancing networks of a 22-type repeater such as a cord circuitrepeater, the impedance of the net- Work ordinarily is given some valuerepresenting an average of the impedances of the lines it is to balance,since thenA lengths of the loops or trunks differ and-.consequently theimpedance works can be so constructed that'they are effec-n tivelyduplicate copies of the actual lines that they must balance, regardlessof the lengths of these lines. Hence, as the repeater `is employedbetween lines of different lengths whose impedances vary, the impedancesof the correspondying networks will vary in the same way, therebymaintaining a close balance between the impedance of the lines and thatof their corresponding balancing networks. This 'makes it possible toobtain much greater -gains from such a repeater` than would be possibleif the networks were fixed, i. e., not variable.

v In the system of Fig. '7 this automatic balancing is done by means-ofthermistors 25, 26, v25v and 2,6', which have .their heater elements H1,

'Ha H1I and H2 inserted in .series with the lines and a. battery such asV155 and 45. As .the length of the line varies, the current, or powerdissipated in these heaters varies 4and the resistance values of theircorresponding temperature-dependent resistance elements T11, T, T11' andT' also vary, these resistance values (with the usual type ofthermistor) becoming rapidly larger as the current through the heatersbecomes less or as the length of the loop or trunk becomes greater.

When the cord circuit is connected between two lines, it is desired thatbefore switches and 5lv are Aclosed to complete the repeaterr circuit,the link 8| automatically be giveny any adjustment required for buildingout the impedance of 1 the connected set Btoznake the Vimpedance ezpresented to terminals 53 of hybrid coil `2 by network N equal the.impedance Z2 presented to terminals 52 of the hybrid coil 2 or balance'the impedance Zz sufficiently closely, and the link 8| automatically begiven any adjustmentrequired for building out the impedance oftheconnected set 8B to -make the impedance z2 presented to the terminals53 of hybrid coil 2 by network N equal the impedance Z2 presented toterminals 52 of the hybrid coil 2 or balance the impedance Z2suiiiciently closely, so the repeater will be prevented from having anyundue singing tendency or any undue limitation on its permissible gain.

The automatic adjustment of the link 8| is effected by response ofthermistor 26 and network 3| to give r the proper value, and response ofthermistor 25, switch 34 and the circuit `therebetween (as described inconnection with Fig. 1) to properly adjust the capacities 33 and 35connected in circuit, so the 1r network formed by the resistance r andthe capacities 33 and 35 will represent closely the actual loop or trunkto which plug IB has been connected. f

The longer the line 6 to which plug I8 is con nected, the greater willbe its resistance, the less will be the current in H2 and the greaterwill be T, Rand r. The responsiveness of T and 3| must, by properdesign, be made such asalways to establish and maintain within thelimits of tolerance the balance between thev resistance component of Z2and that of z2.

The automatic adjustment of the link 8| is similar to that of link 8|.

With the system of Fig. '7, (as with `that ofV 10 Eig. 1), in cases inwhich the variation of length of the lines connected to the repeater, orthe range of impedance Variation of the lines, does not ex- Aceedcertain limits,v the adjustment of the vresistance balance may beadequate for reducing singing tendency and permitting the desired amountof repeater gain, Without necessity for `provision ofv capacities 33 and35 and capacities Y33 and 35'.; and then theseA capacities and theircontrol circuits may` be omitted. However, (as in the case of thecapacities33 and 35 of Fig. 1), the provision of thecapacities 33 and 35enables closer impedance balance between Z2 vand z2 to be obtained, andsimilarly the provision .of capacitiesv 33' and 35" enablescloser'impedance balance between Z2 and 'zz' to be obtained.

If plug .I8 be connected, for example,v to a line that is so longor hassuch high capacity that,

A,with switch 34 in its extreme right-hand position the degree ofbalance between Z2 .and .z2 is within the limits of tolerance, then theresponse of Tn,

s4 and the circuit connecting them (as described in connectionwith'Fig. 1) should be insuiiicient Whereas,v if plug |8Abe connected to'a line so short as to require a given reduction of the capacities33'and 35 in circuit in order to bring the degree of balance between Z2and z2 within the limits of tolerance, then the response of Tn, 34 andtheir connecting circuit should be such as to effect such reduction. Asindicated above, control of capacities 33 and 35 is similar to thecontrol of the capacities 33 and 35.

When the line 6 to which plug |8 is connected is comparatively short,the direct current from battery 45 through heater H9k will be relativelylarge and the resistance of the heated element T9 will be comparativelysmall.l Since T9 is connected across the input circuit of amplierthrough network 87, if desired, the output of this amplifier will bereduced and the amplier gain will be smaller than were the line longerand the resistance of T9 correspondingly higher. This is desirablebecause the longer the line the greater will be its attenuation and thegreater will be the gain required from the amplifier in order to olsetthis greater attenuation and stabilize the transmissionequivalent of thecircuit and the levels at the terminals. If network 81 be omitted, thenwhen the lines 6 to which plug I8 may be connected are long, the simple`shunt bridging loss of T1 may be not such as to give the required shapeto the characteristic of the variation of amplier gain as a function ofthe line length. In this case the required shaping can be obtained witha shaping network such as 81, and if desired a transformer such as thetransformer 32 shown in connection with shaping network 3|, (as theproper variation of 1' as a function of the resistance of T is obtainedwith network 3| and transformer 32, and as the proper variation of thecurrent through coil 3S as a function of the resistance of Te isobtained with the aid of network 61, for eX- ample). As indicated above,the control of the gain of amplifier by thermistor is similar to thecontrol of the .gain of amplifier by ther- K mistor 95.

In the systems of Figs. 1 and 7, the substations or telephone sets maybe of any suitable type. If desired, I2, and I2 may be anti-sidetonetelephone sets of any of the types disclosed in my copending applicationSerial No. 463,184, led October 24, 1942, entitled Telephone system,having the balancing network in the set automatically adjusted inaccordance with the length or impedance of the line to which the set isconnected.

What is claimed is:

l. A wave translating system comprising a line impedance which may havedifferent values, two circuits, electric wave amplifying means having anamplier input circuit connected to one of said two circuits and anamplifier output circuit connected to the other, means providing abalancing circuit for said line impedance and a line circuit for two-waycommunication with said line impedance through said two first-mentionedcircuits and said amplifying means, means comprising said balancingcircuit for said line impedance for connecting saidv two first-mentionedcircuits in energy transmitting relation to said line impedance and inconjugate relation to each other, and thermistor means responsive tochange in the value of said line impedance for changing the impedance ofsaid balancing circuit in substantially the same ratio to maintainconjugacy of said two first-mentioned circuits.

2. Awave translating system comprising a line impedance which may havedifferent values, two circuits, wave amplifying means having anarnplifier input circuit connected to one of said two circuits and anamplifier output circuit connected to the other, means providing abalancing circuit for said line impedance and a line circuit for twowaycommunication with said line impedance through said two iirst-mentionedcircuits and said amplifying means, means comprising said balancingcircuit for said line impedance for connecting said two first-mentionedcircuits in energy transmitting relation to said line impedance and inconjugate relation to each other, impedances comprising atemperature-dependent resistance in said balancing circuit adjustablefor producing changes of the impedance of said balancing circuitcorresponding to changes occurring in said line impedance to maintainbalance of said line impedance by said balancing circuit, and meanscomprising a heating element for said temperature-dependent resistanceresponsive to said changes in said line impedance for effecting saidadjustment of said adjustable impedances.

3. A wave translating system comprising lines of different resistance,two circuits, wave amplifying means having an amplifier input circuitconnected to one of said two first-mentioned circuits and an amplieroutput circuit connected to the other of said two rst-mentionedcircuits, means providing a balancing circuit and a line circuit fortwo-way communications with any selected one of said lines through saidtwo rstmentioned circuits and said amplifying means, said balancingcircuit being adapted to balance the impedance of the selected line,means comprising said balancing circuit for connecting said tworst-mentioned circuits in energy transmitting relation to the selectedline and in conjugate relation to each other, means for producing directcurrent dependent in magnitude on the magnitude of the resistance of theselected line, and means comprising a variable resistance in saidbalancing circuit responsive to said direct current for rendering themagnitude of said variable resistance a function of the magnitude ofsaid direct current.

4. A wave translating system comprising lines of different resistance,two circuits, wave amplifying means having an amplifier input circuitconnected to one of said two circuits and an amplifier output circuitconnected to the other,

means providing a balancing circuit and a line circuit for two-waycommunication with any selected one of said lines through said twoiirstmentioned circuits and said amplifying means, said balancingcircuit being adapted to balance the impedance of the selected line,means comprising said balancing circuit for connecting said twofirst-mentioned circuits in energy transmitting relation to the selectedline and in conjugate relation to each other, means in one of said twofirst-mentioned circuits for supplying direct current to the selectedline, and means responsive to said current for controlling theresistance of said balancing circuit.

5. A wave translating system comprising lines of different resistance,two circuits, wave amplifying means having an amplifier input circuitconnected to one of said two circuits and an amplifier output circuitconnected to the other, means providing a balancing circuit and a linecircuit for two-way communication with any selected one of said linesthrough said two firstmentioned circuits and said amplifying means, saidbalancing circuit being adapted tobalance the impedance of the selectedline, means comprising said balancing circuit for connecting said twoiirst-mentioned circuits in energy transmitting relation to the selectedline and in conjugate relation to each other, said balancing circuitcornprising a four-terminal network having shunt capacity and seriesresistance, means for producing direct current in the selected linedependent in magnitude on its resistance, and means responsive to saidcurrent for varying said capacity and said resistance.

6. A telephone transmission system comprising a group of subscriberscircuits of different resistances, a group of subscribers circuits ofapproximately a given resistance, a 21-type repeater having two pairs ofterminals, a 7r-network comprising adjustable shunt capacities and atemperature-dependent resistancen for connecting one of said pairs toany chosen one of said second group of circuits, means for adjustingsaid capacities, temperature-dependent resistance for controlling saidadjusting means, a source of direct current for energizing saidsubscribers circuits to condition them for operation, current responsiveheating means for said temperaturedependent resistances, and a circuitcomprising said heating means for connecting said other pair ofterminals to any chosen one of said i'lrst group of circuits with saidheating means in series with that chosen circuit and said source.

7. A wave translating system comprising a line impedance whoseresistance component may have diierent magnitudes, two circuits, waveampliiying meanshaving an amplifier input circuit connected to one ofsaid two circuits and an amplifier output circuit connected to theother, means providing a balancing circuit for said line impedance and aline circuit for two-way communication lwith said line impedance throughsaid two first-mentioned circuits and said amplifying means, meanscomprising said balancing circuit for said line impedance for connectingsaid two first-mentioned circuits in energy transmitting relation tosaid line impedance and in conjugate relation to each other, means forproducing direct current dependent in magnitude on the magnitude of theresistance component of said line impedance, said balancing circuitcomprising a four-terminal network terminated in a variable resistance,and means comprising said variable resistance `responsive t0V variationof network has different preassigned values-'for y given values of saidvariable resistance.

8. A wave translating system comprising a line impedance whoseresistance component mayhave different magnitudes, two circuits, waveamplifying means having an amplifier input circuit vconnected to one ofsaid two circuits and an amplifier output circuit connected to theother, means providing a balancing circuit for said line impedance and aline circuit for two-way communication with said line vimpedance throughsaid two first-mentioned circuits and said amplifying means, meanscomprising said balancing circuit for said line impedance for connectingsaid two first-mentioned circuits in energy transmitting relation tosaid line impedance and in conjugate relation to each other, means forproducing in said line impedance a current depending in magnitude on itsresistance `component, said balancing circuit comprising a fourterminalnetwork terminated in a temperaturedependent resistance, and a heatingelement for said temperature-dependent resistance responsive to saidcurrent in said line impedance, said network consisting of a pluralityof resistances of such values and circuit configuration that the ratioof said temperature-dependent resistance to the input resistance of saidnetwork has different preassigned values for given Values of saidtemperature-dependent resistance.

9. A wave translating system comprising a line impedance whoseresistance component may have different magnitudes, two circuits, waveamplifying means having an amplifier input circuit connected to one ofsaid two circuits and an amplifier output circuit connected to theother, means providing a balancing circuit for Said line impedance and aline circuit for two-way communication with said line impedance throughsaid two first-mentioned circuits and said amplifying means, meanscomprising said balancing circuit for said line impedance for connectingsaid two first-mentioned circuits in energy transmitting relation tosaid line impedance and in conjugate relation to each other, means forproducing in said line impedance a current depending in magnitude on itsresistance component, said balancing circuit comprising an adjustableimpedance, means for adjusting said adjustable impedance, afour-terminal network connected to said adjusting means and terminatedin a temperature-dependent resistance for controlling said adjustingmeans, and means responsive to said current in said line impedance forcontrolling said temperature-dependent resistance, said networkconsisting o-f a plurality of resistances f such values and circuitconguration that the ratio of said temperature-dependent resistance tothe resistance of said network facing said adjusting means has differentpreassigned values for given values of said temperature-dependentresistance.

10, A wave translating system comprising lines of diierent resistance,another line, a link circuit for establishing connections of said otherline with any selected one of said first-mentioned lines, a two-wayrepeater in said link circuit having an amplifying path for amplifyingwaves transmitted from the selected line to said other line and anamplifying path for amplifying waves transmitted from said 'other' lineto the Yselected line, a source of electromotive force in said linkcircuit for supplying direct current .to the selected line, and meansresponsive to said direct current for increasing and decreasing-thetransmission efliciency of one of said paths upon increase anddecrease,respectively, ci the resistance of the selected line by change of theline selection. Y v i 11. In a telephone system, a plurality ofsubscriber lines of different resist-ences, a connecting circuit forestablishing connections with said lines, a two-way repeater invsaidconnecting circut having an amplifying path for receiving and amplifyingwaves ircm'said lines and an amplifying path for amplifying wavesandtransmitting the waves so amplified to said lines, a source ofelectromotive force in said connecting circuit for supplying directcurrent to the line with vwhich connection is established, meanscomprising a resistance of high temperature coefficient in said firstpath, and av heating element therefor electrically insulated therefromincluded in said connecting circuit and responsive to the direct currentsupplied to the line. Y t

12. A wave translating system comprising a line, two circuits, meanscomprising a balancing circuit for said line fo-r connecting said twocircuits in energy transmitting relation to said line and inconjugaterelation to each other, said bal- Aancing circuit comprising anadjustable impedance, means for adjusting said impedance, a circuitconnected to said adjusting means and comprising a resistance ofnegative temperature coeincient for controlling said adjusting means, aheating element for said resistance electrically insulated therefrom, asecond resistance of negative temperature coeiicient, means forproducing decreasing current in said heating element in response todecrease of said second resistance, and a heating element for saidsecond resistance electrically insulated therefrom and responsive tocurrent in said line for controlling said second resistance.

13. A system for'transrnitting current changes comprising an inputcircuit, an output circuit, means for producing variable input currentin said input circuit and means for producing correspondingly variableoutput current in said output circuit, said last means comprising aresistance of high temperature coefcient connected in said outputcircuit, a heating element therefor electrically insulated therefrom, asecond resistance of high temperature coeicient of the same sign as therst coefficient, a heating element for said second resistance connectedin said input circuit, a source of constant voltage for supplyingheating energy to said first heating element, and a circuit comprisingsaid second resistance connecting said source to said first heatingelement to supply energy of said source thereto and responsive tomagnitude change of one sign in current in said second heating elementfor producing magnitude change of opposite sign in current in said rstheatingelement.

14. A control circuit comprising a resistance of high temperaturecoefficient, a heating element therefor electrically insulatedtherefrom, a second resistance of high temperature coeiiicient of thesame sign as the first coefcient, a heating element for saidsecondvresistance electrically insulated therefrom, a source ofelectromotive force for supplying energy to heat said first heatingelement, and means comprising said second resistance connecting saidsource to supply heating energy to said first heating element andresponsive to change of current of one sense in said second heatingelement for producing current change of opposite sense in said iirstheating element.

l5. A system comprising a temperature-,dependent resistance, a heatingelement therefor electrically insulated therefrom, a secondtemperature-dependent resistance having its temperature coecient ofresistance of the same sign as that of said rst resistance, a heatingelement for said second resistance electrically insulated therefrom, asource of electromotive force for supplying energy to heat said firstheating element, means comprising said second resistance and havingvariable transmission eiciency connecting said source to said rstheating element to supply energy of said source thereto and ren sponsivet0 change of current of one sign in said second heating element for sovvarying said transmission efficiency as to produce current change ofopposite sign in said rst heating element, and

means comprising a second source of electrometive force in circuit withsaid second heating element adapted to produce therein current change ofmagnitude sufficient to produce temperature change of said firstresistance large compared to its maximum temperature Change produced byambient temperature.

16. In combination, a resistance of high temperature coefficient, aheating element therefor electrically insulated therefrom, a secondresistance of high temperature coeflicient of the same sign as the rstcoeflicient, a heating element for said second resistance electricallyinsulated therefrom, av source of electronic-tive force of givenfrequency, and a circuit comprising in series said source, said firstheating element and a network Vtuned to approximately said frequency,said network comprising a capacity and in parallel therewith said secondresistance and an inductance in series.

KENNETH S. JOHNSON.

